The rotor blades of a rotorcraft are subject to air vortices during operation. These give rise to noise and vibrations that stand in the way of the acceptance of rotorcraft by the general public. Moreover, such vibrations are disadvantageous in terms of service life and maintenance since they can lead to material fatigue of the parts and to constant relative movements of the parts, with the resultant wear and tear.
The aero-mechanical and aero-elastic phenomena—for instance, the collision of a rotor blade with blade vortices coming from the preceding rotor blade and the resulting forces that act upon the rotor blade—that can be observed during the landing approach at certain angles of descent of the rotorcraft cause vibrations and lead to intermittent noise patterns.
It is likewise known that the approach stream conditions acting on a rotor of a rotorcraft change constantly, even in the stationary flight state. These changes result from the overlapping of the forward speed of the rotorcraft with the rotational speed of the rotor.
This superimposition essentially gives rise to two extreme approach stream situations, namely, at the forward blade and at the aft blade. At the forward blade, the rotational component is positively superimposed on the translational component and this leads to an approach stream having a highly transonic characteristic at the tip of the blade. In this case, the thinnest possible profile with little curvature is desired in order to avoid strong supersonic bangs. At the aft blade, the translational component of the forward airspeed is subtracted from the rotational component. This results in a great reduction in the local approach stream velocity, even causing the approach stream to strike the profile from the rear in the inner area of the blade. In conjunction with the high angle of incidence of the blade that prevails in the aft blade area, the low relative approach stream velocity causes stalling of the flow, which has a negative effect on the dynamic push-pull rod loads and on the power requirements of the rotor. A thicker profile or a profile with a pronounced curvature would be advantageous in this area.